1. Field of the Invention
This invention relates to an objective lens driving device useable with an information reading-out apparatus which projects a light spot onto a record medium which has been spirally or concentrically recorded with one or more information tracks and reads-out the information. The objective lens is driven in a direction perpendicular to its optical axis and the information track for the purpose of correcting relative positional error, i.e. tracking error, between the information track and the light spot.
2. Description of the Prior Art
The above mentioned information reading-out apparatus is well known and the record medium including the information track is usually called a video disc. In this video disc, encoded video and audio signals are recorded as optical information by means of optical transmitting properties, reflection phase properties, or the like. The video disc is rotated at a high speed and a laser beam emitted from a laser light source is focussed through an objective lens onto the information track and the reflected or transmitted light modulated by the information track is detected and read out. An important property of such a record medium is the very high density of recorded information possible and thus the width of the information track is very narrow and the space between successive tracks is also very narrow. Therefore, the diameter of the light spot should be made correspondingly small. In order to pick up correctly the recorded information from such tracks having very narrow width and pitch, the relative positional error between the light spot and the information track, i.e. the tracking error, should be made as small as possible.
It has been proposed to effect so-called tracking control in which the relative displacement of the light spot and the information track is photoelectrically detected to produce a tracking error signal and the light spot is moved in a direction perpendicular to the track in response to the detected tracking error signal.
As such a tracking mechanism, it has been proposed to use an oscillating mirror inserted in an optical path from a laser light source to an objective lens, and to rotate the mirror in response to the detected tracking error signal. However, such a tracking mechanism could not attain sufficiently high accuracy and response and is liable to be large and expensive.
In order to avoid the above-mentioned drawbacks of known tracking mechanisms, it has been further proposed to support the objective lens or its holding frame by a resilient supporting member composed of leaf springs and to move the objective lens in a direction perpendicular to its optical axis as well as to the tangential direction of the information track by means of an electromechanical transducer in response to the tracking error signal. In such a tracking mechanism which makes use of leaf springs, the electro-mechanical transducer may be an electromagnet, voice coil, piezoelectric element or the like. In order to obtain a good response property for tracking, the assembly should be made small and lightweight. In an actual reading-out apparatus, in addition to the tracking error a so-called focussing error is produced in which the light spot could not be correctly focussed on the information track. For correcting the focussing error, a focussing mechanism must be provided for displacing the objective lens in the direction of its optical axis. When the tracking mechanism is mounted on the focussing mechanism, the tracking mechanism should be smaller and lighter in weight for attaining accurate focussing correction.
In a tracking mechanism with an electromagnet, a sufficient force necessary for moving the objective lens can be produced and the mechanism can be made small and lightweight in a relatively easy manner. However, accurate tracking correction could not be effected, because the relation between the amount of electric current passing through a coil of the electromagnet and the amount of displacement of the objective lens is not linear. In the tracking mechanism comprising a voice coil, miniaturization could hardly be attained. Further, the tracking mechanism with the piezoelectric elements could not produce sufficiently large force for driving the objective lens.